Synthetic peptides corresponding to V/beta chain sequences commonly expressed by encephalitogenic T cells specific for basic protein (BP) or proteolipid protein (PLP) can induce specific cellular and humoral immunity that can protect and treat rats and mice developing paralytic experimental encephalomyelitis. The focus of the current proposal is to address the biological significance of this powerful regulatory mechanism, including 1) why germline TCR sequences are recognized immunologically as foreign even though TCR chains are surely present within the thymic environment. 2) how TCR peptide specific T cells and antibodies recognize and regulate pathogenic T cells. and 3) how immunoregulation occurs in vivo? To address these questions, the following Specific Aims are proposed. Aim 1. To determine the immunological basis for adult recognition of TCR peptides. Two testable alternatives will be evaluated to explain the immunogenicity of the V/beta8.2-39-59 peptide, including 1) that this idiotope represents a "cryptic determinant distinct from tolerogenic forms produced during thymic processing of TCR molecules and 2) that TCR idiotopes expressed with MHC on thymic T cells - especially immature CD4+CD8+ T cells - promote positive selection of TCR peptide reactive T cells rather than negative selection typical for self antigens presented by thymic accessory cells. Aim 2. To determine the mechanism of immunoregulation of V/beta8.2+ T cells by anti-TCR peptide specific T cells and antibodies. To characterize the immunogenic form of the V/beta8.2-39-59 idiotope, the processing and expression of TCR and MHC molecules on V/beta8.2- T cell clones and hybrids recognized by anti-V/beta8.2-39-59 reactive T cells and antibodies will be inhibited by monoclonal antibodies, brefeldin A, and anti-sense DNA. Moreover, supernatants from TCR peptide specific T cells, show previously to inhibit activation and encephalitogenicity of V/beta8.2+ BP reactive T cells will be evaluated for the presence of known inhibitory lymphokines (lL-4, lL-10, & TGF-beta). Aim 3. To determine what immunological changes occur in TCR peptide treated rats. Using parental to F1 transfers, where donor T cells can be distinguished by allelic markers, the fate and function of encephalitogenic and regulatory T cells will be followed in vivo. Aim 4. To induce CD8+ TCR peptide-specific cytotoxic T cells. Proteoliposome-encapsulated peptides will be used to try to induce cytotoxic T cells to delete rather than regulate encephalitogenic V/beta8.2+ T cells in vivo information that will emerge from this proposal is crucial to the understanding and clinical application of idiotypic immunoregulation directed at TCR and potentially other non- tolerogenic cellular determinants.